Abu Sa'id al-Khudri reported God’s Messenger as saying, “When allegiance is sworn to two caliphs kill the second of them.*
Muslim transmitted it.
* Some interpret ‘kill’ here as meaning to fight with. The tradition is directed against the possibility of two Caliphs holding power at the same time, even though the areas might be quite distinct.
Abu Huraira reported the Prophet as saying, “The B. Isra’il were governed by the prophets, as often as one died another taking his place. There will be no prophet after me, but there will be numerous caliphs.” He was asked what command he had to give and replied, “Fulfil the oath of allegiance to each and give them their due for God will question them about what He asked them to guard.”
‘Arfaja told that he heard God’s Messenger say, “Various corruptions will arise, so strike with the sword him who tries to cause separation in this people when they are united, whoever he be.”
‘Abdallah b. ‘Umar told that he heard God's Messenger say, “He who throws off obedience will meet God on the day of resurrection without possessing any plea, and he who dies without having taken an oath of allegiance will die like a pagan.”